Field of the Invention
The present invention pertains generally to ophthalmic surgery, which is useful for correcting vision deficiencies. More particularly, the present invention pertains to systems and methods for precise depth measurements of corneal layers within the eye.
Corneal shape corrective surgeries are commonly used to treat myopia, hyperopia, astigmatism, and the like. Procedures employing an excimer laser include laser assisted in-situ keratomileusis (LASIK), photo refractive keratectomy (PRK) and laser sub-epithelial keratomileusis (LASEK). During LASIK, a suction ring is typically placed over sclera tissue (the white part of the eye) to firmly hold the eye. A microkeratome with an oscillating steel blade can be used to make a partial incision through the front surface of a cornea and/or to automatically pass across the cornea to create a thin flap of tissue on the front central part of the eye. Alternatively, a femtosecond pulsed laser beam may be used to create a corneal flap. After the suction ring is removed, the flap is lifted to expose tissue for ablation with a laser. The laser is typically programmed to correct a desired amount of visual effect, and directs a laser beam at the exposed tissue. A rapid emission of laser pulses removes very small precise amounts of corneal tissue. In LASIK, one objective is the removal of only stromal tissue with the consequent preservation of anterior corneal tissue (e.g., preservation of the epithelium and Bowman's layer). After irrigation with saline solution, the corneal flap is folded back to heal in the pre-procedure or original position.
The flap incision is typically made to a depth below Bowman's layer, for example, to ensure exposure of the stroma when the flap is lifted back. Bowman's layer may be difficult to identify, so many conventional procedures incise the flap at a pre-determined constant depth or distance from the anterior surface of the cornea or optionally from an aplanation lens that might be used to contact the cornea. This depth or distance may be derived from a historical or population based average of corneal thicknesses and may also include a buffer depth. Because some corneas have an irregular thickness profile, some of these incision depths are conservatively pre-determined and can result in deep incisions into the stroma that incise more stromal tissue than is typically needed to form the flap. A remaining stromal layer (i.e., the flap bed following the flap incision) that is too thin may interfere with a desired ablation of the stroma for vision correction.
In light of the above, it would be desirable to provide systems, apparatus, and methods for accurately measuring depths within the cornea for vision correction procedures. It would also be desirable to provide accurate depth measurements for use with producing corneal flap incisions below Bowman's layer while preserving or maximizing the amount of remaining stromal material (e.g., in the flap bed) for vision correction.